Method and system for finding tooth features on a virtual three-dimensional model

ABSTRACT

A method and system are disclosed for finding virtual tooth features on virtual three-dimensional models of the teeth of patients. The tooth features comprise marginal ridges, cusp tips, contact points, central groove and buccal groove. Tooth axes system plays a key role in identifying the tooth features. An iterative method is disclosed for improving the accuracy of the tooth axes system. A virtual three-dimension model preferably obtained by scanning the dentition of a patient forms the basis for determining the tooth features. Tooth features are derived for all categories of teeth including molars, premolars, canines and front teeth. Tooth features are very helpful and used in planning orthodontic treatment. The tooth features are determined automatically using the computerized techniques; and can be manually adjusted when necessary.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application entitled “METHOD ANDSYSTEM FOR COMPREHENSIVE EVALUATION OF ORTHODONTIC CARE USING UNIFIEDWORKSTATION,” Ser. No. 11/133,996, filed May 20, 2005, pending, theentire contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

A. Field of the Invention

This invention relates to the field of computerized techniques fororthodontic treatment planning for human patients. More particularly,the invention is directed to finding virtual tooth features, which arevery helpful and used in planning orthodontic treatment, on a virtualthree-dimensional model of dentition. The tooth features are determinedautomatically using the computerized techniques; and can be manuallyadjusted when necessary.

B. Description of Related Art

The traditional process of diagnosis and treatment planning for apatient with orthodontic problems or disease typically consists of thepractitioner obtaining clinical history, medical history, dentalhistory, and orthodontic history of the patient supplemented by 2Dphotographs, 2D radiographic images, CT scans, 2D and 3D scanned images,ultrasonic scanned images, and in general non-invasive and sometimesinvasive images, plus video, audio, and a variety of communicationrecords. Additionally, physical models, such as made from plaster ofparis, of the patient's teeth are created from the impressions taken ofthe patient's upper and lower jaws. Often, such models are manuallyconverted into teeth drawings by projecting teeth on drawing paper.Thus, there is a large volume of images and data involved in thediagnosis and treatment planning process. Furthermore, the informationmay require conversion from one form to another and selective reductionbefore it could become useful. There are some computerized toolsavailable to aid the practitioner in these data conversion and reductionsteps, for example to convert cephalometric x-rays (i.e., 2 dimensionalx-ray photographs showing a lateral view of the head and jaws, includingteeth) into points of interest with respect to soft tissue, hard tissue,etc., but they are limited in their functionalities and scope. Eventhen, there is a fairly substantial amount of manual work involved inthese steps.

Consequently, the practitioner is left to mental visualization, chanceprocess to select the treatment course that would supposedly work.Furthermore, the diagnosis process is some-what ad-hoc and theeffectiveness of the treatment depends heavily upon the practitioner'slevel of experience. Often, due to the complexities of the detailedsteps and the time consuming nature of them, some practitioners take ashortcut, relying predominantly on their intuition to select a treatmentplan. For example, the diagnosis and treatment planning is often done bythe practitioner on a sheet of acetate placed over the X-rays. All ofthese factors frequently contribute towards trial and error,hit-and-miss, lengthy and inefficient treatment plans that requirenumerous mid-course adjustments. While at the beginning of treatmentthings generally run well as all teeth start to move at least into theright direction, at the end of treatment a lot of time is lost byadaptations and corrections required due to the fact that the end resulthas not been properly planned at any point of time. By and large, thisapproach lacks reliability, reproducibility and precision. More over,there is no comprehensive way available to a practitioner to stage andsimulate the treatment process in advance of the actual implementationto avoid the often hidden pitfalls. And the patient has no choice anddoes not know that treatment time could be significantly reduced ifproper planning was done.

In recent years, computer-based approaches have been proposed for aidingorthodontists in their practice. See Andreiko, U.S. Pat. No. 6,015,289;Snow, U.S. Pat. No. 6,068,482; Kopelmann et al., U.S. Pat. No.6,099,314; Doyle, et al., U.S. Pat. No. 5,879,158; Wu et al., U.S. Pat.No. 5,338,198, and Chisti et al., U.S. Pat. Nos. 5,975,893 and6,227,850, the contents of each of which is incorporated by referenceherein. Also see imaging and diagnostic software and other relatedproducts marketed by Dolphin Imaging, 6641 Independence Avenue, CanogaPark, Calif. 91303-2944.

U.S. Pat. No. 6,648,640 to Rubbert, et al. describes an interactive,computer based orthodontist treatment planning, appliance design andappliance manufacturing. A scanner is described which acquires images ofthe dentition, which are converted to three-dimensional frames of data.The data from the several frames are registered to each other to providea complete three-dimensional virtual model of the dentition. Individualtooth objects are obtained from the virtual model. Acomputer-interactive software program provides for treatment planning,diagnosis and appliance design from the virtual tooth models. A desiredocclusion for the patient is obtained from the treatment planningsoftware. The virtual model of the desired occlusion and the virtualmodel of the original dentition provide a base of information for custommanufacture of an orthodontic appliance. A variety of possible applianceand appliance manufacturing systems are contemplated, includingcustomized arch wires and customized devices for placement of off-theshelf brackets on the patient's dentition for housing the arch wires,and removable orthodontic appliances.

U.S. Pat. No. 6,632,089 to Rubbert, et al. describes an interactive,software-based treatment planning method to correct a malocclusion. Themethod can be performed on an orthodontic workstation in a clinic or ata remote location such as a lab or precision appliance-manufacturingcenter. The workstation stores a virtual three-dimensional model of thedentition of a patient and patient records. The virtual model ismanipulated by the user to define a target situation for the patient,including a target arch-form and individual tooth positions in thearch-form. Parameters for an orthodontic appliance, such as the locationof orthodontic brackets and resulting shape of an orthodontic arch wire,are obtained from the simulation of tooth movement to the targetsituation and the placement position of virtual brackets. The treatmentplanning can also be executed remotely by a precision appliance servicecenter having access to the virtual model of the dentition. In thelatter situation, the proposed treatment plan is sent to the clinic forreview, and modification or approval by the orthodontist. The method issuitable for other orthodontic appliance systems, including removableappliances such as transparent aligning trays.

Other background references related to capturing three dimensionalmodels of dentition and associated craniofacial structures include S. M.Yamany and A. A. Farag, “A System for Human Jaw Modeling UsingIntra-Oral Images” in Proc. IEEE Eng. Med. Biol. Soc. (EMBS) Conf., Vol.20, Hong Kong, October 1998, pp. 563-566; and M. Yamany, A. A. Farag,David Tasman, A. G. Farman, “A 3-D Reconstruction System for the HumanJaw Using a Sequence of Optical Images,” IEEE Transactions on MedicalImaging, Vol. 19, No. 5, May 2000, pp. 538-547. The contents of thesereferences are incorporated by reference herein.

The technical literature further includes a body of literaturedescribing the creation of 3D models of faces from photographs, andcomputerized facial animation and morphable modeling of faces. See,e.g., Pighin et al., Synthesizing Realistic Facial Expression fromPhotographs, Computer Graphics Proceedings SIGGRAPH '98, pp. 78-94(1998); Pighin et al., Realistic Facial Animation Using Image-based 3DMorphing, Technical Report no. UW-CSE-97-01-03, University of Washington(May 9, 1997); and Blantz et al., A Morphable Model for The Synthesis of3D Faces, Computer Graphics Proceedings SIGGRAPH '99 (August, 1999). Thecontents of these references are incorporated by reference herein.

Tooth features, such as the cusp tips, marginal ridges, central groovelines, buccal grooves, contact points, tooth axes system, etc. play keyroles in defining some well established orthodontic treatment planningcriteria such as: alignment, marginal ridges, buccolingual inclination,occlusal relationships, occlusal contacts, overjet, interproximalcontacts, root angulation, etc. Indeed, the American Board ofOrthodontics (ABO) has introduced an Objective Grading System (OGS) forevaluating the results of an orthodontic treatment once it is completedusing these criteria. Alignment refers to an assessment of toothalignment. In the anterior region, the incisal edges and lingualsurfaces of the maxillary anterior teeth and the incisal edges andlabial-incisal surfaces of the mandibular anterior teeth are chosen toassess anterior alignment. In the maxillary posterior region, themesiodistal central groove of the premolars and molars is used to assessadequacy of alignment. In the mandibular arch, the buccal cusps of thepremolars and molars are used to assess proper alignment. Marginalridges are used to assess proper vertical positioning of the posteriorteeth. If marginal ridges are at the same height, it will be easier toestablish proper occlusal contacts, since some marginal ridges providecontact areas for opposing cusps. Buccolingual inclination is used toassess the buccolingual angulation of the posterior teeth. In order toestablish proper occlusion in maximum intercuspation and avoid balancinginterferences, there should not be a significant difference between theheights of the buccal and lingual cusps of the maxillary and mandibularmolars and premolars. Occlusal relationship is used to assess therelative anteroposterior position of the maxillary and mandibularposterior teeth. The buccal cusps of the maxillary molars, premolars,and canines must properly align with the interproximal embrasures of themandibular posterior teeth. The mesiobuccal cusp of the maxillary firstmolar must properly align with the buccal groove of the mandibular firstmolar. Occlusal contacts are measured to assess the adequacy of theposterior occlusion. Again, a major objective of orthodontic treatmentis to establish maximum intercuspation of opposing teeth. Therefore, thefunctioning cusps are used to assess the adequacy of this criterion;i.e., the buccal cusps of the mandibular molars and premolars, and thelingual cusps of the maxillary molars and premolars. Overjet is used toassess the relative transverse relationship of the posterior teeth, andthe anteroposterior relationship of the anterior teeth. In the posteriorregion, the mandibular buccal cusps and maxillary lingual cusps are usedto determine proper position within the fossae of the opposing arch. Inthe anterior region, the mandibular incisal edges should be in contactwith the lingual surfaces of the maxillary anterior teeth. Interproximalcontacts are used to determine if all spaces within the dental arch havebeen closed. Persistent spaces between teeth after orthodontic therapyare not only unesthetic, but can lead to food impaction. Root angulationis used to assess how well the roots of the teeth have been positionedrelative to one another.

Traditionally, the tooth features discussed above are visuallyidentified and marked by the practitioner; and various measurementsrelated to the treatment criteria are performed manually using measuringinstruments and gauges. The ABO has developed an orthodontic measuringgauge to assist in the manual measurement of parameters related to theOGS criteria discussed above from the dental cast and the panoramicradiograph. Although the measuring gauges introduce a degree ofconsistency in the measurements when performed by different people, themeasurements are still limited in scope to two-dimensional analysis.

Therefore, in order to enable computerized orthodontic treatmentplanning, and three-dimensional, accurate measurements of the criteriasuch as those developed by ABO, there is a need for digitally findingtooth features, such as the cusp tips, marginal ridges, central groovelines, buccal grooves, contact points, tooth axes system, etc., on athree-dimensional virtual dentition model of a patient.

U.S. Pat. No. 6,616,444 to Andreiko, et al. describes a system andmethod by which an orthodontic appliance is automatically designed andmanufactured from digital lower jaw and tooth shape data of a patient.The method provides for scanning a model of the patient's mouth toproduce two or three dimensional images and digitizing contours andselected points. A computer may be programmed to construct archformsand/or to calculate finish positions of the teeth, then to design anappliance to move the teeth to the calculated positions.

U.S. Pat. No. 6,322,359 to Jordan, et al. describes a computerimplemented method of creating a dental model for use in dentalarticulation. The method provides a first set of digital datacorresponding to an upper arch image of at least a portion of an upperdental arch of a patient, a second set of digital data corresponding toa lower arch image of at least a portion of a lower dental arch of thepatient, and hinge axis data representative of the spatial orientationof at least one of the upper and lower dental arches relative to acondylar axis of the patient. A reference hinge axis is created relativeto the upper and lower arch images based on the hinge axis data.Further, the method may include bite alignment data for use in aligningthe lower and upper arch images. Yet further, the method may includeproviding data associated with condyle geometry of the patient, so as toprovide limitations on the movement of at least the lower arch imagewhen the arch images are displayed. Further, a wobbling technique may beused to determine an occlusal position of the lower and upper dentalarches. Various computer implemented methods of dental articulation arealso described. For example, such dental articulation methods mayinclude moving at least one of the upper and lower arch images tosimulate relative movement of one of the upper and lower dental archesof the patient, may include displaying another image with the upper andlower dental arches of the dental articulation model, and/or may includeplaying back recorded motion of a patient's mandible using the dentalarticulation model.

The invention disclosed herein offers a novel method and system fordigitally finding tooth features, such as the cusp tips, marginalridges, central groove lines, buccal grooves, contact points, tooth axessystem, etc., on a three-dimensional virtual dentition model of apatient.

SUMMARY OF THE INVENTION

The invention is directed to finding virtual tooth features on virtualthree-dimensional models of teeth. The tooth features comprise marginalridges, cusp tips, contact points, central groove and buccal groove. Avirtual three-dimension model preferably obtained by scanning thedentition of a patient forms the basis for determining the toothfeatures. Tooth features are derived for all categories of teethincluding molars, premolars, canines and front teeth. Tooth features arevery helpful and used in planning orthodontic treatment. The toothfeatures are determined automatically using the computerized techniques;and can be manually adjusted when necessary. The tooth axes system (TAS)plays an important role in determining the tooth features listed above.Indeed, the TAS it self is considered a tooth feature. The TAS iscreated for each individual tooth based up on the ideal properties ofthe tooth in terms of its features. TAS is preferably an anatomicalcoordinate system for the tooth. It is preferably derived during virtualmodelling of the tooth from the scanning data using the tooth templates.The TAS for a virtual tooth comprises the origin, the x-axis in themesial and distal directions, the y-axis in the buccal and lingualdirections and the z-axis in the occlusal and gingival (vertical)directions.

In a first aspect of the invention, a method is disclosed for findingthe ridge on a virtual three-dimensional model of a molar or a premolar.The ridge comprises a set of outer most points on the occlusal surfaceof the tooth determined as follows:

(a) a set of sufficient number of curves is produced along the surfaceof the tooth by intersecting the tooth surface with planes containingthe z-axis from the TAS of the virtual tooth;

(b) each curve is then traced starting from the bottom of the tooth (theface opposite to the occlusal surface) first on one side (e.g. left) andthen the other side and a ridge point is found on either side such thatthe tangent to the tooth surface at the ridge point is preferablyhorizontal or nearly horizontal; and

(c) the family of points obtained in this manner forms the ridge for thetooth.

In another aspect of the invention, a method is disclosed for findingthe marginal ridges on virtual three-dimensional models of molars andpremolars. Two local minima, one at the mesial side of the tooth, andanother at the distal side of the tooth are found from the ridge. Thesetwo minima are the marginal ridges for the tooth.

In another aspect of the invention, a method is disclosed for findingthe cusp tips on virtual three-dimensional models of molars andpremolars. The marginal ridges are used to divide the ridge into twosubsets, with each subset bounded by the two marginal ridges, one subsetat the buccal side of the tooth (the buccal branch), and the other atthe lingual side (the lingual branch). Now the local maxima (maximalz-coordinate) of the buccal branch define the buccal cusp tips, and thelocal maxima of the lingual branch define the lingual cusp tips.

In another aspect of the invention, a method is disclosed for findingthe cusp tips on virtual three-dimensional models of canines. The mostor highest occlusal point of the virtual tooth model (the point with thehighest z-coordinate) is chosen as the cusp tip for a canine tooth.

In another aspect of the invention, a method is disclosed for findingthe cusp tips on virtual three-dimensional models of front teeth. Foreach front tooth, two points of the occlusal lateral edges are definedas the “cusp tips”. With regard to these cusp tips, there is adifferentiation made between maxilla and mandible front teeth. In themaxilla these two points are situated on the lingual side of the frontteeth; while in the mandible they are situated on the labial side of thefront teeth. In order to determine these cusp tips, the tooth model isrotated around the tooth TAS origin or any other point in the tooth,first by 45° in a) mesial direction, or b) distal direction; and afterthat in c) lingual direction, or d) labial direction. This wholeprocedure is repeated twice, once with direction a), then with directionb). This leads to the two points for the cusp tips. Direction d) ischosen for the maxilla teeth, direction c) for the mandible teeth. Afterthe rotation, the most or highest occlusal point of the rotated toothmodel is chosen as a cusp tip.

In another aspect of the invention, a method is disclosed for findingthe ideal contact points on virtual three-dimensional models of teeth.The surface of the tooth model is intersected with the x-y plane basedup on the TAS for the tooth. Then, the most mesial point and the mostdistal point (i.e. points with the lowest/highest x-coordinates), arethe two ideal contact points for the tooth.

In another aspect of the invention, a method is disclosed for correctingthe TAS. TAS plays a key role in identifying the tooth features. Aniterative method is disclosed for improving the accuracy of the toothaxes system. The cusp tips are used to correct the TAS for a tooth asfollows:

a) If there are at least three cusp tips, then the TAS is rotated insuch a way that all cusp tips have (approximately) the samez-coordinate. If there are three cusp tips, they will have exactly thesame z-coordinate. On the other hand, if there are more than three cusptips, then an approximate a solution is found which minimizes thedifferences of the z-coordinates.

b) If there are two cusp tips (such as in premolars), then the TAS isrotated around an axis, which passes through the origin of the TAS forthe tooth and is perpendicular to the z-axis and to the vectorconnecting the two cusp tips, by an angle such that the z-coordinates ofthe two cusp tips are equal.

In another aspect, when the TAS is corrected in a manner describedabove, the ridge, the marginal ridges and the cusp tips are determinedagain in order to improve the accuracy of the tooth features.Subsequently, the TAS is also refined, and the entire process ofrefining the tooth features is repeated. The process may be stopped when(a) a limit is reached in the angle of rotation of the correctionapplied to the TAS, or (b) the maximum iteration count is reached.Alternately, a combination of the iteration stopping criteria (a) and(b) can also be used.

In another aspect of the invention, a method is disclosed for findingthe central groove on virtual three-dimensional models of molars andpremolars. The most occlusal (i.e. the point having the highestz-coordinate) cusp tips of the lingual side C^(l) and the buccal sideC^(b) are determined using the procedure described earlier. Then, thetooth is rotated around the TAS x-axis using a rotation matrix, so thatC^(l) and C^(b) have the same z-coordinates. Next, a plane E_(k)parallel to the y-z-plane is moved and cut with all edges of the toothsurface which is represented in the form of triangles derived during theregistration process of the scanned tooth model, of which bothvertex-normals point upwards. Then, the intersection points are sortedalong the y-axis. The tooth surface contour shows the typical shape of amolar with two “mountains” and a “valley” (the central groove). Fromeach slice of the tooth surface, the deepest point of the valley isfound. The criterion specified above assures that only the edges of theocclusal surface of the tooth are considered. Next, a linearapproximation of the points is found, i.e. a (three-dimensional) line,which has minimal average (quadratic) distance to the points. This linedoes not need to be the exact minimum, an approximation is good enough.Then, the two points c₀ and c₁ are calculated as the end-points of thecentral groove of the tooth. It should be noted that these centralgroove points c₀ and c₁ are in general not on the surface of the toothmodel. Therefore, for proper use of the central groove in treatmentplanning, the central groove points are put onto the tooth surface. Onepossibility for accomplishing this is to construct a line through c₀ andc₁, respectively, parallel to the z-axis of the TAS for the tooth (i.e.the occlusal direction), and cut it with all triangles of the toothmodels. Of all intersection points, the most occlusal point (i.e. thepoint with the greatest z-coordinate) on the surface of the tooth istaken as the point representing the central groove. That is, theprocedure is done once for c₀ and again for c₁.

In another aspect of the invention, a method is disclosed for findingthe buccal groove on virtual three-dimensional models of molar. Theprocedure is summarized as follows:

Step 1. Let {tilde over (C)}_(M) be the mesial labial cusp tip of thetooth-model of interest, and {tilde over (C)}_(D) the distal labial cusptip. Then, find the two closest vertices C_(M) and C_(D).

Step 2. Rotate the tooth-model around its x-axis (torque it) linguallyby 60°.

Step 3. After this rotation, rotate the tooth-model around an axis,which is orthogonal to the z-axis and to the vector connecting the twocusps C_(M) and C_(D) so that the two cusps have the same z-coordinate.Move the cusp tips along the tooth-surface, until they are on localmaxima, and repeat rotation for each iteration of Step 3.

a) Perfom rotation;

b) Choose one of the two cusp tips, which has been moved less up to now(or any, if they have been moved equally).

c) Let N(C) be the neighborhood of C, i.e. all vertices of the toothmodel, which share an edge with C. From N(C), select a vertex C′ with,i.e. the neighboring point of C, which has the greatest z-coordinateafter applying the transformation. If C_(z)′≦C_(z), this means that C isalready a local maximum. In this case choose the other cusp (that means,now C will stand for C_(D) if it stood for C_(M) before and vice versa),and repeat step c). If both cusps are local maxima, then step 3) isfinished; and go to step 4).

d) replace C by C′ and go back to a).

Step 4. Find a “saddle-point” (the “lowest” vertex along the “highest”path from cusp to cusp):

(that means, always take the point with the greatest z-coordinate (afterapplying the transformation) of all vertices, which meet iii) and iv))

From this path, choose the vertex S with the smallest z-coordinate.Then, S is the buccal groove of the tooth model under consideration.

The path, which connects the “moved cusp tips”, located at theend-points of the path. The path is the highest possible path, whichconnects the cusp tips. The lowest point along this path is the“saddle-point”, which is the buccal grove of the virtual tooth.

BRIEF DESCRIPTION OF THE DRAWINGS

Presently preferred embodiments of the invention are described below inreference to the appended drawings, wherein like reference numeralsrefer to like elements in the various views, and in which:

FIG. 1 is an illustration of an orthodontic care system incorporating ahand-held scanner system. The hand-held scanner is used by theorthodontist or the assistant to acquire three-dimensional informationof the dentition and associated anatomical structures of a patient andprovide a base of information to diagnose and plan treatment for thepatient.

FIG. 2 is an illustration of a patient being scanned with a hand-heldscanner.

FIGS. 3A and 3B display a front view and an occlusal view, respectively,of a three-dimensional model of the complete maxilla dentition of apatient created by the scanning and registration process.

FIG. 4A-4F are a series of illustrations showing the generation of anindividual tooth model from a scanned tooth, shown in FIG. 4A, and atemplate tooth, shown in FIG. 4B. A library of template teeth similar toFIG. 4A are stored as three-dimensional computer models in computermemory. The individual tooth model is a three-dimensional tooth objecthaving a single set of points defining the boundaries of the tooth.Individual tooth models are also invaluable in interactive orthodontictreatment planning since they can be independently moved relative toeach other in simulation of treatment scenarios.

FIG. 5 is an illustration of the tooth model of FIG. 4D positioned inthe computer model of the patient's dentition, surrounded by otheranatomical structures. FIG. 6 is a screen shot displaying a buccal viewof the individual tooth objects created from the maxilla jaw of apatient using the tooth separation process. FIG. 7A shows the tooth axessystem for a virtual tooth as an example.

FIG. 7B shows an example of the tooth axes system for each tooth in themaxilla of a patient.

FIG. 8 shows a screen shot illustrating the process for finding theridge on a virtual tooth surface, according to a preferred embodiment ofthe invention.

FIG. 9 shows a screen shot illustrating the ridge on a virtual toothmodel surface, according to a preferred embodiment of the invention.

FIG. 10 illustrates a screen shot showing the marginal ridges on avirtual tooth, according to a preferred embodiment of the invention.

FIG. 11 illustrates a screen shot showing marginal ridges on a group ofteeth, according to a preferred embodiment of the invention.

FIG. 12 illustrates another screen shot showing marginal ridges on agroup of teeth, according to a preferred embodiment of the invention.

FIG. 13 shows a screen shot illustrating the marginal ridges and thecusp tips on the virtual model of a tooth, according to a preferredembodiment of the invention.

FIG. 14 shows a screen shot illustrating the single cusp tip of a caninetooth, according to a preferred embodiment of the invention.

FIG. 15 shows a screen shot illustrating the two cusp tips of an upperfront tooth, according to a preferred embodiment of the invention.

FIG. 16 shows another screen shot illustrating the two cusp tips of anupper front tooth, according to a preferred embodiment of the invention.In FIG. 16, the tooth was rotated by 45° to distal direction and 45° tolabial direction. In this situation the left cusp tip was found as themost occlusal point.

FIG. 17 shows a screen shot illustrating the ideal contact points on apatient's teeth in maxilla, according to a preferred embodiment of theinvention.

FIG. 18 shows a screen shot illustrating a tooth model presented in adistal view showing the lingual cusp tips and the buccal cusp tips,according to a preferred embodiment of the invention. The cusp tips areexaggerated in FIG. 18 for the purpose of better illustration.

FIG. 19 shows a screen shot illustrating the virtual tooth model from adistal view, as shown in FIG. 18, after the rotation, according to apreferred embodiment of the invention. In this figure, the buccal cusptips and the lingual cusp tips of the tooth have the same z-coordinate(i.e. the coordinate in the occlusal direction).

FIG. 20 shows a screen shot illustrating one intersection (or one slice)of the virtual tooth model shown in FIG. 19, according to a preferredembodiment of the invention. The tooth surface contour shows the typicalshape of a molar with two “mountains” and a “valley” (the centralgroove).

FIG. 21 shows a screen shot illustrating he points which are the deepestpoints, one of each slice, found in the process of determining thecentral groove, according to a preferred embodiment of the invention.The figure also shows the linear approximation of these points.

FIG. 22 shows a screen shot illustrating the tooth shown in FIG. 21 fromanother perspective, according to a preferred embodiment of theinvention.

FIG. 23 shows a screen shot illustrating central grooves for severalteeth models in the maxilla of a patient, according to a preferredembodiment of the invention.

FIG. 24 shows another screen shot illustrating central grooves forseveral teeth models, according to a preferred embodiment of theinvention.

FIG. 25 shows a screen shot illustrating a tooth model from the distalview perspective in the process of determining the buccal grove;according to a preferred embodiment of the invention.

FIG. 26 shows a screen shot illustrating the tooth model, shown in FIG.25, rotated 60 degrees to the labial side so that the labial side isnearly on top, according to a preferred embodiment of the invention.

FIG. 27 shows a screen shot illustrating the tooth model, now fromlabial perspective, according to a preferred embodiment of theinvention. Now the contours show two hills with a valley in between. Thevalley is the buccal groove.

FIG. 28 shows a screen shot illustrating the tooth model after applyingthe second rotation, so that the two hills have the same height,according to a preferred embodiment of the invention.

FIG. 29 shows a screen shot illustrating the tooth model with the cusptips. The smooth lines show the way of the “moving” cusp tips, accordingto the preferred embodiment of the invention.

FIG. 30 shows a screen shot illustrating the virtual tooth model and the“saddle point”, according to the preferred embodiment of the invention.

FIGS. 31 and 32 show screen shots illustrating several tooth models withbuccal groves, according to the preferred embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Overview

FIG. 1 is an illustration of an orthodontic care system 10 incorporatinga scanner system 12. The scanner system 12 includes a hand-held scanner14 that is used by the orthodontist or his assistant to acquirethree-dimensional information of the dentition and associated anatomicalstructures of a patient. The images are processed in a scanning node orworkstation 16 having a central processing unit, such as ageneral-purpose computer. The scanning node 16, either alone or incombination with a back-office server 28, generates a three-dimensionalcomputer model 18 of the dentition and provides the orthodontist with abase of information for diagnosis, planning treatment, and monitoringcare for the patient. The model 18 is displayed to the user on a monitor20 connected to the scanning node 16.

As noted above, the scanner system 12 described in detail herein isoptimized for in-vivo scanning of teeth, or alternatively, scanning aplaster model of the teeth and/or an impression of the teeth.

The orthodontic care system consists of a plurality of orthodonticclinics 22 which are linked via the Internet or other suitablecommunications medium 24 (such as the public switched telephone network,cable network, etc.) to a precision appliance service center 26. Eachclinic 22 has a back office server work station 28 having its own userinterface, including a monitor 30. The back office server 28 executes anorthodontic treatment planning software program. The software obtainsthe three-dimensional digital data of the patient's teeth from thescanning node 16 and displays the model 18 for the orthodontist. Thetreatment planning software includes features to enable the orthodontistto manipulate the model 18 to plan treatment for the patient. Forexample, the orthodontist can select an archform for the teeth andmanipulate individual tooth positions relative to the archform to arriveat a desired or target situation for the patient. The software moves thevirtual teeth in accordance with the selections of the orthodontist. Thesoftware also allows the orthodontist to selectively place virtualbrackets on the tooth models and design a customized archwire for thepatient given the selected bracket positions. When the orthodontist hasfinished designing the orthodontic appliance for the patient, digitalinformation regarding the patient, the malocclusion, and a desiredtreatment plan for the patient are sent over the communications mediumto the appliance service center 26. A customized orthodontic archwireand a device for placement of the brackets on the teeth at the selectedlocation is manufactured at the service center and shipped to the clinic22.

As shown in FIG. 1, the precision appliance service center 26 includes acentral server 32, an arch wire manufacturing system 34 and a bracketplacement manufacturing system 36. For more details on these aspects ofthe illustrated orthodontic care system, the interested reader isdirected to the patent application of Rüdger Rubbert et al., filed Apr.13, 2001, entitled INTERACTIVE AND ARCHWIRE-BASED ORTHODONTIC CARESYSTEM BASED ON INTRA-ORAL SCANNING OF TEETH, Ser. No. 09/835,039, nowissued as U.S. Pat. No. 6,648,640, the entire contents of which areincorporated by reference herein.

Capture of Image Information

FIG. 2 is an illustration of a patient 70 being scanned with thehand-held scanner 14. The checks and lips are retracted from the teethand the tip 68 of the scanner is moved over all the surfaces of theteeth in a sweeping motion. The entire upper or lower jaw may need to bescanned in a series of scans. While FIG. 2 illustrates in-vivo scanningof a human patient, the scanner can of course be used to scan a plastermodel of the dentition if that is preferred, or an impression taken fromthe patient. It is also possible that a scan of a patient may bepartially taken in vivo and the remainder from a model or an impression.

Basically, during operation of the scanner to scan an object of unknownsurface configuration, hundreds or thousands of images are generated ofthe projection pattern as reflected off of the object in rapidsuccession as the scanner and the object are moved relative to eachother. For each image, pixel locations for specific portions, i.e.,points, of the reflected pattern are compared to entries in the scannercalibration table. X, Y and Z coordinates (i.e., three dimensionalcoordinates) are obtained for each of these specific portions of thereflected pattern. For each picture, the sum total of all of these X, Yand Z coordinates for specific points in the reflected pattern comprisea three-dimensional “frame” or virtual model of the object. Whenhundreds or thousands of images of the object are obtained fromdifferent perspectives, as the scanner is moved relative to the object,the system generates hundreds or thousands of these frames. These framesare then registered to each other to thereby generate a complete andhighly accurate three-dimensional model of the object. The scanning andthe registration processes are described in detail in the patentapplication of Rüdger Rubbert, et al., filed Apr. 13, 2001, entitledMETHODS FOR REGISTRATION OF THREE-DIMENSIONAL FRAMES TO CREATETHREE-DIMENSIONAL VIRTUAL MODELS OF OBJECTS, Ser. No. 09/835,007,pending, the entire contents of which are incorporated by referenceherein.

FIG. 3A displays a front view of a three-dimensional model of the upperjaw 100 of the dentition of a patient created through the scanning andregistration process described above.

FIG. 3B displays an occlusal view 200 of the three-dimensional model ofthe upper jaw shown in FIG. 3A.

Separation of Teeth into Individual Tooth Objects (Tooth Modeling)

FIGS. 4A-4F are a series of illustrations showing the generation of anindividual tooth model from a scanned tooth. The process is summarizedbelow.

FIG. 4A shows the scanned dentition and associated anatomical structuresurrounding the tooth 308. The back office server workstation stores athree-dimensional virtual template tooth object for each tooth in themaxilla and the mandible. The template tooth 310 for the tooth number308 is shown in FIG. 4B. The template tooth object 310 is athree-dimensional tooth object having a single set of points definingthe boundaries of the tooth. As shown in FIG. 4C, the template tooth 310is positioned approximately in the same location in space as the tooth308. The landmark 302 assists in providing the proper axial rotation ofthe template tooth to have it fit properly with respect to the tooth308. The template tooth is placed at the point cloud of the dentitionaccording to the labial landmark 302. The template tooth can be scaledlarger or smaller or positioned arbitrarily by the user, in order to geta close a position as possible to the point cloud of the dentition.

As shown in FIG. 4D, vectors are drawn from the points on the templatetooth to the scanned point cloud of the tooth 308. Every ray intersectsseveral surfaces, depending on how often the respective part of thesurface has been covered during scanning. For each vector, a surface isselected. Preferably, the smallest triangle surface is selected, sincethis surface corresponds to an image taken by the scanner when thescanner was positioned in a more perpendicular orientation to thedentition surface, resulting in more accuracy in the determination ofthe coordinates of that portion of the surface. As another possibility,the outermost surface is selected, using a filter to insure that noextraneous surfaces are used. These points of the surfaces intersectedby all the vectors are combined as newly generated triangle surfaces andtherefore form one consistent surface shown in FIG. 4E. Then, finally,missing parts of the tooth are completed from the template tooth. Theresult is shown in FIG. 4F. In a second pass, this generated object isthen used as a template tooth, and the steps indicated by FIG. 4C-4F arerepeated in an iterative fashion. This is done to make sure that thealgorithm works even if there are significant differences between theoriginal template tooth and the scanned point cloud, e.g, a gap in scandata, different geometry in the tooth. The goal is to provide analgorithm that does not require a closely fitting template tooth object.

The final result, an individual three-dimensional virtual tooth object312, is then displayed to the user, as shown in FIG. 5. The result maybe displayed on the workstation user interface as a three-dimensionalsuperposition of the original data (white) and the separated model ofthe tooth (darker tones or contrasting color). These tones allow theuser to ascertain whether there is an even distribution of white anddark tones, indicating good fit between the scanned tooth 308 and theindividual tooth object 312. This step may be automated by an algorithmdetecting the difference (or the sum of the differences), and repeatingthe process if the difference is too great.

Separation of teeth from the virtual model of the dentition could alsobe performed automatically using algorithms to detect incisal edges ofthe teeth, grooves between teeth, and grooves indicating theintersection of the gums and the teeth. Two types of errors can occurwhen separation of teeth objects from other structure (e.g., other teethand gums): 1) the data is selected for a tooth that does not inactuality belong to the tooth, such as gums and adjacent teeth, and 2)data that does belong to the tooth is ignored. The entire processincluding these and other aspects is described in greater detail in thepatent application of Rüdger Rubbert, et al., filed Apr. 13, 2001,entitled METHOD AND WORKSTATION FOR GENERATING VIRTUAL TOOTH MODELS FROMTHREE-DIMENSIONAL TOOTH DATA, Ser. No. 09/834,413, pending, the entirecontents of which are incorporated by reference herein.

This process is of course performed for all the teeth. The result is aset of individual tooth objects for all the teeth in the patient'sdentition. The teeth can be displayed either alone, or in conjunctionwith the surrounding anatomical structures such as shown in FIG. 5.

FIG. 6 is a screen shot displaying a buccal view 400 of the individualtooth objects 410 created from the maxilla jaw of a patient using thetooth separation process described above.

The tooth model, once created, can be modified to simulate varioustreatments that may be made on the tooth.

Treatment Planning

The virtual model of the patient's dentition, and the individual toothobjects created as explained above, provide a base for diagnosticanalysis of the dentition and treatment planning. Treatment planningsoftware is provided on the workstation of the orthodontic clinic, andpossibly at other remote locations such as the precision appliancecenter of FIG. 1. The treatment planning software can be considered aninteractive, computer-based computer aided design and computer aidedmanufacturing (CAD/CAM) system for orthodontics. The apparatus is highlyinteractive, in that it provides the orthodontist with the opportunityto both observe and analyze the current stage of the patient'scondition, and to develop and specify a target or desired stage. Ashortest direct path of tooth movement to the target stage can also bedetermined. Further, the apparatus provides for simulation of toothmovement between current and target stages. For further details ontreatment planning, refer to the previously mentioned patent applicationof Rüdger Rubbert et al., filed Apr. 13, 2001, entitled INTERACTIVE ANDARCHWIRE-BASED ORTHODONTIC CARE SYSTEM BASED ON INTRA-ORAL SCANNING OFTEETH, Ser. No. 09/835,039, now issued as U.S. Pat. No. 6,648,640, theentire contents of which are incorporated by reference herein.

Tooth features, such as the cusp tips, marginal ridges, central groovelines, buccal grooves, contact points, etc. play key roles in definingsome well established orthodontic treatment planning criteria such as:alignment, marginal ridges, buccolingual inclination, occlusalrelationships, occlusal contacts, interproximal contacts, rootangulation, etc. Indeed, the American Board of Orthodontics (ABO) hasintroduced an Objective Grading System (OGS) for evaluating the resultsof an orthodontic treatment once it is completed using these criteria.

Tooth Features

Methods for digitally finding the tooth features, such as the tooth axessystem, marginal ridges, cusp tips, contact points, central groovelines, and buccal grooves on a virtual three-dimensional model of atooth, according to preferred embodiments of the invention, will now bedescribed.

Tooth Axes System

FIG. 7A shows the tooth axes system (TAS) 450 for the virtual tooth 440as an example. TAS 450 comprises the origin 452, the x-axis 454 in themesial and distal directions, the y-axis 456 in the buccal and lingualdirections and the z-axis 458 in the occlusal and gingival (vertical)directions. TAS is created for each individual tooth based up on theideal properties of the tooth in terms of its features; and is notderived from the jaw features. TAS is preferably an anatomicalcoordinate system for the tooth. It is preferably derived during virtualmodelling of the tooth from the scanning data and the tooth templates,and is adjusted if necessary.

FIG. 7B shows an example of the TAS 480 for each tooth in the maxilla ofa patient. The y-axis and the z-axis of each TAS are shown in FIG. 7B;however the x-axis is not shown for simplicity sake. TAS for a toothbehaves independently of the TAS for other teeth. Orientation of TAS isas such not fixed in space. Indeed TAS for any tooth moves with thetooth, and its position is determined by the position of the tooth. TASplays an important role in tooth position measurements.

Rotation Matrix

A 3×3 rotation matrix M^(rot)(v,α) is defined so that for xεR³

-   -   M^(rot)(v,α)·x is x rotated around the axis v by the angle α per        Eq. (1);

where, v must have length 1 (one), i.e. √{square root over (v_(x)²+v_(y) ²+v_(z) ²)}=1. $\begin{matrix}{{M^{rot}\left( {v,\alpha} \right)}:=\begin{pmatrix}{{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{x} \cdot v_{x}} + {\cos\quad\alpha}} & {{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{x} \cdot v_{y}} - {{v_{z} \cdot \sin}\quad\alpha}} & {{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{x} \cdot v_{z}} + {{v_{y} \cdot \sin}\quad\alpha}} \\{{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{y} \cdot v_{x}} + {{v_{z} \cdot \sin}\quad\alpha}} & {{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{y} \cdot v_{y}} + {\cos\quad\alpha}} & {{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{y} \cdot v_{z}} - {{v_{x} \cdot \sin}\quad\alpha}} \\{{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{z} \cdot v_{x}} - {{v_{y} \cdot \sin}\quad\alpha}} & {{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{z} \cdot v_{y}} + {{v_{x} \cdot \sin}\quad\alpha}} & {{\left( {1 - {\cos\quad\alpha}} \right) \cdot v_{z} \cdot v_{z}} + {\cos\quad\alpha}}\end{pmatrix}} & {{Eq}.\quad(1)}\end{matrix}$

The procedures for finding several tooth features require toothrotations at desired angles. The tooth rotation is accomplished by usingEq. (1).

Ridge for Molars and Pre-Molars

The process of finding some of the tooth features, according to apreferred embodiment of the invention, starts with finding the ridge onthe occlusal side of the tooth model surface for molars and premolars.

FIG. 8 shows a screen shot illustrating the process for finding theridge on a tooth surface. First, a plane 510 is defined containing thez-axis from the TAS of the virtual tooth 500 under consideration suchthat the plane intersects the entire surface of the tooth model andforms a curve 512 along the surface of the tooth model. Next, the curve512 is traced starting from the bottom of the tooth (the face oppositeto the occlusal surface) first on one side (e.g. left) with the help ofa moving tangent 514 and then the other side (e.g. right) with the helpof a moving tangent 516 applied to the curve. When the tangent 514becomes horizontal as the tangent 520 (i.e., having the vanishingz-coordinate) it designates the ridge point 524 on the curve andconsequently on the surface of the tooth; and the tracing process withrespect to this curve is stopped in this direction. Alternately, thecurve tracing process may be stopped when the tangent becomes close tohorizontal as defined by an arbitrary threshold, for example 5 degrees.The tangent 516 in the other direction leads to the horizontal tangent522 and the ridge point 526. Two ridge points 524 and 526 on the surfaceof the tooth are thus found by tracing the curve. These two ridge points524 and 526 with respect to the plane 510 are recorded. The plane isthen rotated by a certain angle (for example 15 degrees) using Eq. (1),while still containing the z-axis of the tooth, and it intersects thetooth surface producing another curve along the surface of the toothmodel. The process of tracing the curve is repeated and another set oftwo ridge points on the surface of the tooth are found. The process ofrotating the plane and tracing the curve is repeated until the entiresurface of the tooth model has been explored. For each position of theplane a set of two ridge points on the surface of the tooth model arefound. The two ridge points in any set will (normally) be on oppositesides of the z-axis. As illustrated by FIG. 9, this family of the ridgepoints defines the ridge 530 of the virtual tooth 500.

Marginal Ridges of Molars and Premolars

Marginal ridges are the tooth features for molars and premolars.According to a preferred embodiment of the invention, as illustrated inFIG. 10, two local minima 532 and 534, one at the mesial side of thetooth, and another at the distal side of the tooth are found from theridge family of points 530 for the virtual tooth 500. These two minima532 and 534 are the marginal ridges for the tooth.

FIGS. 11 and 12 illustrate screen shots showing marginal ridges 540 on agroup of teeth. Lines 542 connecting the marginal ridges are only shownfor illustrative purposes.

There are different ways possible for defining these two minima. In oneaspect, the tooth surface is divided into two halves, one with positivex-coordinates, and another with negative. In each of these halves theabsolute minimum of the ridge is taken as a marginal ridge. In anotheraspect, the whole tooth model is rotated around its y-axis by 45degrees, once in each of the two directions, each time searching theabsolute minimum of the rotated ridge. One skilled in the art wouldappreciate that other approaches for determining the marginal ridges arepossible as well.

Cusp Tips of Molars and Premolars

The procedure for digitally finding the cusp tips for molars andpremolars is explained with reference to FIG. 13, according to apreferred embodiment of the invention. In order to determine the cusptips for a virtual tooth model 500, the marginal ridges 532 and 534 areused to divide the ridge 530 into two subsets 552 and 554, with eachsubset bounded by the two marginal ridges, one subset 552 at the buccalside of the tooth (the buccal branch), and the other 554 at the lingualside (the lingual branch). Now the local maxima (maximal z-coordinate)of the buccal branch define the buccal cusp tips 562 and the localmaxima of the lingual branch define the lingual cusp tips 564.

If there are more local maxima along a branch than expected (accordingto the tooth morphology), then the most distinct ones are chosen. Oneskilled in the art would appreciate that there are differentpossibilities for defining the ‘distinctness’. One preferred method isto find the preceding and the succeeding local minimum for each localmaximum, and measure their ‘distance’ (this distance can be defined asthe Euclidian distance, or as the angle between the intersecting planesused in finding the ridge), on which these mimima are situated.

Correction of Tooth Coordinate System

The cusp tips can be used to correct the TAS for a tooth, according to apreferred embodiment of the invention, as follows:

a) If there are at least three cusp tips, then the TAS is rotated insuch a way that all cusp tips have (approximately) the samez-coordinate. If there are three cusp tips, they will have exactly thesame z-coordinate. On the other hand, if there are more than three cusptips, then an approximate a solution is found which minimizes thedifferences of the z-coordinates.

b) If there are two cusp tips (such as in premolars), then the TAS isrotated around an axis, which passes through the origin of the TAS forthe tooth and is perpendicular to the z-axis and to the vectorconnecting the two cusp tips, by an angle such that the z-coordinates ofthe two cusp tips are equal.

In one aspect, when the TAS is corrected in a manner described above,the ridge, the marginal ridges and the cusp tips are determined again inorder to improve the accuracy of the tooth features. Subsequently, theTAS is also refined, and the entire process of refining the toothfeatures is repeated. The process may be stopped when (a) a limit isreached in the angle of rotation of the correction applied to the TAS,or (b) the maximum iteration count is reached. Alternately, acombination of the iteration stopping criteria (a) and (b) can also beused.

Cusp Tips of Canines

Canines only have one cusp tip. The procedure for finding the cusp tipfor a canine is illustrated with reference to FIG. 14, according to apreferred embodiment of the invention. The most or highest occlusalpoint 610 of the virtual tooth model 600 (the point with the highestz-coordinate) is chosen as the cusp tip for a canine tooth.

Cusp Tips of Front Teeth

For each front tooth, two points of the occlusal lateral edges aredefined as the “cusp tips” for the purpose of an embodiment of theinvention disclosed herein. With regard to these cusp tips, there is adifferentiation made between maxilla and mandible front teeth. In themaxilla these two points are situated on the lingual side of the frontteeth; while in the mandible they are situated on the labial side of thefront teeth.

In order to determine these cusp tips, according to a preferredembodiment of the invention, the tooth model is rotated using Eq. (1)around the tooth TAS origin or any other point in the tooth, first by45° in a) mesial direction, or b) distal direction; and after that in c)lingual direction, or d) labial direction. This whole procedure isrepeated twice, once with direction a), then with direction b). Thisleads to the two points for the cusp tips. Direction d) is chosen forthe maxilla teeth, direction c) for the mandible teeth.

After the rotation, the most or highest occlusal point of the rotatedtooth model is chosen as a cusp tip.

One skilled in the art would appreciate that other approaches, insteadof taking the most occlusal point at the cusp tip, are also possiblefinding the cusp tips for the front teeth. For example, in anotherembodiment of the invention, the centroid of the area comprising allpoints of the tooth model, which fulfills the condition that itsz-coordinate differs not more than a certain threshold from the mostocclusal point of the rotated tooth model, is chosen as the desired cusptip.

FIG. 15 shows a screen shot illustrating the two cusp tips 630 of anupper front tooth 620.

FIG. 16 shows another screen shot illustrating the two cusp tips 640 ofan upper front tooth 635. In FIG. 16, the tooth was rotated by 45° todistal direction and 45° to labial direction. In this situation the leftcusp tip was found as the most occlusal point.

Ideal Contact Points

In order to find the ideal contact points for a virtual tooth, accordingto a preferred embodiment of the invention, the surface of the toothmodel is intersected with the x-y plane based up on the TAS for thetooth. Then, the most mesial point and the most distal point (i.e.points with the lowest/highest x-coordinates), are the two ideal contactpoints for the tooth.

FIG. 17 shows a screen shot illustrating the ideal contact points 650 ona patient's teeth in maxilla.

Central Groove

In order to determine the central groove for a tooth model, according toa preferred embodiment of the invention, the most occlusal (i.e. thepoint having the highest z-coordinate) cusp tips C^(l) (=(C^(l) _(x),C^(l) _(y), C^(l) _(z))) of the lingual side, and C^(b) (=(C^(b) _(x),C^(b) _(y), C^(b) _(z))) of the buccal side are determined using theprocedure described earlier.

FIG. 18 shows a screen shot illustrating a tooth model 700 presented ina distal view showing the lingual cusp tips 710 and the buccal cusp tips720. The cusp tips are exaggerated in FIG. 18 for the purpose of betterillustration.

Then, the tooth is rotated around the x-axis using the rotation matrixdefined in Eq. (1), so that C^(l) and C^(b) have the same z-coordinates.This operation in effect means that the following transformation isapplied.$\left. v\mapsto v^{\prime} \right.:={{M^{rot}\left( {\begin{pmatrix}1 \\0 \\0\end{pmatrix},\alpha} \right)} \cdot v}$ with$\alpha:={{\tan^{- 1}\left( \frac{C_{z}^{1} - C_{z}^{b}}{C_{y}^{1} - C_{y}^{b}} \right)}.}$

FIG. 19 shows a screen shot illustrating the virtual tooth model 700from a distal view, shown in FIG. 18, after the rotation. In thisfigure, the buccal cusp tips 720 and the lingual cusp tips 710 of thetooth have the same z-coordinate (i.e. the coordinate in the occlusaldirection). Again the cusp tips 710 and 720 are exaggerated forillustration purposes.

Next, a plane E_(k) parallel to the y-z-plane is moved and cut with alledges of the tooth surface which is represented in the form of trianglesderived during the registration process of the scanned tooth model, ofwhich both vertex-normals point upwards. Then, the intersection pointsare sorted along the y-axis.

FIG. 20 shows a screen shot illustrating one intersection (or one slice)of the virtual tooth model 700 shown in FIG. 19. The tooth surfacecontour 740 shows the typical shape of a molar with two “mountains” 750and a “valley” 760 (the central groove). From each slice of the toothsurface, the deepest point of the valley is found using the followingprocedure.

Planes are selected as: E_(k): v_(x)=k·d with d=0.4 (for example) andkεN

The selection criterion is: A vertex normal n points upwards iffn_(z)>0.5 (for example)

For each plane E_(k) there is a series (v^(k,i))_(1=1, . . . ,I) _(k)with _(y) ^(k,i)≦v_(y) ^(k,i+1) ∀iε{1, . . . ,I_(k)},

which are the intersection points of plane E_(k) with the edges of thetooth-model. The criterion specified above assures that only the edgesof the occlusal surface of the tooth are considered.

To each v^(k,i) one then finds:

m_(<) ^(k,i):=max {v_(z) ^(k,j),j=1, . . . i}, the maximum z-coordinatelingual of v^(k,i),

m_(>) ^(k,i):=max {v_(z) ^(k,j),j=i, . . . I_(k)}, the maximumz-coordinate labial of v^(k,i),

p^(k,i):=(m_(<) ^(k,i)−v_(z) ^(k,i))·(m_(>) ^(k,i)−v_(z) ^(k,i)), theproduct of the differences between the z-coordinate of v^(k,i) and theabove two maxima.

Then, for each k an index i₀ ^(k) is found so that

p^(k,i) ⁰ ^(k) =max {p^(k,i), i=1, . . . ,I_(k)} is the maximum of theseproducts.

Next, referring to FIGS. 21 and 22, a linear approximation l:x

(y,z) of the points 780 (v^(k,i) ⁰ ^(k) )_(k) is found, i.e. a(three-dimensional) line 790, which has minimal average (quadratic)distance to the points v^(k,i) ⁰ ^(k) . This line does not need to bethe exact minimum, an approximation is good enough.

Using the two x-valuesx_(min):=min {v_(x) ^(k,i) ⁰ ^(k) , all k}x_(max):=max {v_(x) ^(k,i) ⁰ ^(k) , all k}

the following two points, c₀ and c₁ as defined below, are the end-pointsof the central groove 790 of the tooth 700.c₀:=(x_(min),l(x_(min)))c₁:=(x_(max),l(x_(max)))

FIG. 22 shows a screen shot illustrating the tooth 700 shown in FIG. 21from another perspective.

FIG. 23 shows a screen shot illustrating central grooves 790 for severalteeth models in the maxilla of a patient.

FIG. 24 shows another screen shot illustrating central grooves 790 forseveral teeth models.

It should be noted that these central groove points c₀ and c₁ are ingeneral not on the surface of the tooth model. Therefore, for proper useof the central groove in treatment planning, it is recommended to putthe central groove points onto the tooth surface. One possibility foraccomplishing this is to construct a line through c₀ and c₁,respectively, parallel to the z-axis of the TAS for the tooth (i.e. theocclusal direction), and cut it with all triangles of the tooth models.Of all intersection points, the most occlusal point (i.e. the point withthe greatest z-coordinate) on the surface of the tooth is taken as thepoint representing the central groove. That is, the procedure is doneonce for c₀ and again for c₁.

Buccal Groove

The process of finding the buccal grove for a tooth model, according toa preferred embodiment of the invention, is described below.

Step 1. Let {tilde over (C)}_(M) be the mesial labial cusp tip of thetooth-model of interest, and {tilde over (C)}_(D) the distal labial cusptip.

Then, find the two closest vertices C_(M) and C_(D), both εT={T₀, . . ., T_(N)}⊂R³ (T is the set of all vertices of the tooth-model, N is thenumber of vertices of the tooth-model):C_(M):=T_(di M)εT with i_(M)ε{1, . . . ,N} so, that dist({tilde over(C)}_(M),T_(i) _(M) )=min {dist({tilde over (C)}_(M),T_(i)),iε{1, . . .,N}}C_(D):=T_(i) _(D) εT with i_(D)ε{1, . . . ,N} so, that dist({tilde over(C)}_(D),T_(i) _(D) )=min {dist({tilde over (C)}_(D),T_(i)),iε{1, . . .,N}}

(where dist(P,Q):=∥P−Q∥:=√{square root over((P_(x)−Q_(x))²+(P_(y)−Q_(y))²+(P_(z)−Q_(z))²)} is the Euclideandistance between two points.)

It should be noted that i_(M) and i_(D) are not always unique. If thisis the case, simply one possible solution is chosen, for example thesolution with the lowest index.

FIG. 25 shows a screen shot illustrating a tooth model 800 from thedistal view perspective.

Step 2. Rotate the tooth-model around its x-axis (torque it) linguallyby 60° using the following transformation matrix, using Eq. (1),$M^{T}:={M^{rot}\left( {\begin{pmatrix}1 \\0 \\0\end{pmatrix},60} \right)}$

FIG. 26 shows a screen shot illustrating the tooth model 800, shown inFIG. 25, rotated 60 degrees to the labial side so that the labial sideis nearly on top.

Step 3. After this rotation, rotate the tooth-model around an axis,which is orthogonal to the z-axis and to the vector connecting the twocusps C_(M) and C_(D) so that the two cusps have the same z-coordinate(accomplished by performing the rotation R* described below). Move thecusp tips along the tooth-surface, until they are on local maxima, andrepeat rotation R* for each iteration of Step 3.

a) Perfom rotation R*:${{{Axis}\text{:}\quad v}:=\left( {\begin{pmatrix}0 \\0 \\1\end{pmatrix} \times \left( {C_{D} - C_{M}} \right)} \right)_{0}},$where $w_{0}:=\frac{w}{w}$is the unit vector (vector with length 1).${{Angle}\text{:}\quad\alpha}:={\sin^{- 1}\left( \frac{\left( {M^{T} \cdot C_{D}} \right)_{z} - \left( {M^{T} \cdot C_{M}} \right)_{z}}{{C_{D} - C_{M}}} \right)}$

Apply rotation to our transformation matrix M^(T):M^(T)←M^(rot)(v,α)·M^(T)

b) Choose one of the two cusp tips, which has been moved less up to now(or any, if they have been moved equally). So, below, C will stand forC_(M) if dist(C_(M),{tilde over (C)}_(M))<dist(C_(D),{tilde over(C)}_(D)), or for C_(D) otherwise.

c) Let N(C) be the neighborhood of C, i.e. all vertices of the toothmodel, which share an edge with C (i.e., which are referenced by atriangle on the tooth surface, which also references C).

From N(C), select a vertex C′ with C_(z)′=max {(M^(T)·P)_(z),PεN(C)},i.e. the neighboring point of C, which has the greatest z-coordinate(after applying the

M^(T) transformation defined above). If C_(z)′≦C_(z), this means that Cis already a local maximum. In this case choose the other cusp (thatmeans, now C will stand for C_(D) if it stood for C_(M) before and viceversa), and repeat step c). If both cusps are local maxima, then step 3)is finished; and go to step 4).

d) replace C by C′ and go back to a).

FIG. 27 shows a screen shot illustrating the tooth model 800, now fromlabial perspective. Now the contours show two hills 810 with a valley820 in between. The valley 820 is the buccal groove.

FIG. 28 shows a screen shot illustrating the tooth model 800, afterapplying the second rotation, so that the two hills 810 have the sameheight.

FIG. 29 shows a screen shot illustrating the tooth model 800 with thecusp tips 830. The smooth lines 840 show the way of the “moving” cusptips. That is, the cusp tips are moved along the surface of the tooth,always along the edge of a triangle on the surface of the tooth, withthe greatest increment of the z-coordinate, until a local maximum isreached, i.e. no more increment in the z-coordinate is possible.

Step 4. Find a “saddle-point” (the “lowest” vertex along the “highest”path from cusp to cusp):

Find a path P₀, . . . , P_(n), with:

i) ∀i: P_(i)εT (the path consists of vertices of the tooth model);

ii) P₀:=C_(M) and P_(n):=C_(D) (the path starts at one cusp tip, andends at the other);

iii) ∀i: P_(i+1)⊂N(P_(i)) (each two consecutive points of the path areconnected by an edge of the tooth model);

iv) ∀iε{2, . . . ,n}, ∀j<i−1:P_(i)∉N(P_(j)) (this condition implies thatthe path contains no loops);

v) ∀iε{1, . . . ,n}: (P_(i))_(z)=max {(M^(T)·P)_(z), PεN(P_(i−1))

P∉N(P₀)

. . .

P∉N (P_(i−2))}

(that means, always take the point with the greatest z-coordinate (afterapplying the transformation) of all vertices, which meet iii) and iv))

From this path, choose the vertex with the smallest z-coordinate, i.e.S:=P_(i′) with i′ so, that (P_(i′))_(z)=min {(M^(T)·P)_(z),iε{0, . . .,n}}.

Then, S is the buccal groove of the tooth model under consideration.

FIG. 30 shows a screen shot illustrating the tooth model 800 and thelast part of the procedure, i.e. finding the “saddle point” 850. Thepath 840, which connects the “moved cusp tips” 830, located at theend-points of the path 840. The path 840 is the highest possible path,which connects the cusp tips 830. The lowest point along this path isthe “saddle-point” 850, which is the buccal grove of the virtual tooth800.

FIGS. 31 and 32 show a screen shots illustrating several tooth modelswith buccal groves 850.

Presently preferred and alternative embodiments of the invention havebeen set forth. Variation from the preferred and alternative embodimentsmay be made without departure from the scope and spirit of thisinvention.

1. A method of finding virtual tooth features on a virtualthree-dimensional model of a tooth, comprising the steps of: (a) findinga ridge; and (b) finding the tooth feature on the virtualthree-dimensional model of a tooth.
 2. The method of claim 1, whereinsaid ridge comprises a set of outer most points on the occlusal surfaceof the tooth, and the step (a) of finding a ridge comprises the stepsof: (i) finding a set of sufficient number of curves produced along thesurface of the virtual model of tooth by intersecting the tooth surfacewith planes containing the z-axis from the tooth axes system of thevirtual tooth; (b) tracing each curve starting from the bottom of thetooth (the face opposite to the occlusal surface) first on one side(e.g. left) and then the other side and there by finding a ridge pointon either side such that the tangent to the tooth surface at the ridgepoint is preferably horizontal or nearly horizontal; and (c) finding afamily of ridge points there by forming the ridge for the tooth.
 3. Themethod of claim 2, wherein the tooth feature is a set of marginalridges, and the step (b) of finding the tooth feature comprises the stepof finding the two local minima, one at the mesial side of the tooth,and another at the distal side of the tooth, from the ridge, where bythese two minima are the marginal ridges for the tooth.
 4. The method ofclaim 3, wherein the tooth feature is a set of cusp tips, and the step(b) of finding the tooth feature further comprises the step of dividingthe ridge into two subsets, wherein each subset is bounded by the twomarginal ridges, one subset at the buccal side of the tooth (the buccalbranch), and the other at the lingual side (the lingual branch), andwherein the local maxima (maximal z-coordinate) of the buccal branchdefine the buccal cusp tips, and the local maxima of the lingual branchdefine the lingual cusp tips.
 5. The method of claim 1, wherein thevirtual tooth is a molar.
 6. The method of claim 1, wherein the virtualtooth is a premolar.
 7. A method of finding the cusp tip on a virtualthree-dimensional model of a canine, comprising the steps of: findingthe highest occlusal point of the virtual tooth model, wherein thehighest occlusal point has the highest z-coordinate; and designating thehighest occlusal point as the cusp tip for the canine.
 8. A method offinding the cusp tips on a virtual three-dimensional model of a fronttooth, comprising the steps of: (a) rotating the tooth model around thetooth axes system origin by 45° in a first direction; where by placingthe tooth model in the first position; (b) rotating the tooth model inthe first position around the tooth axes system origin by 45° in asecond direction, where by placing the tooth model in the secondposition; and (c) selecting the cusp tip, wherein the cusp tip is thehighest occlusal point of the tooth model in the second position.
 9. Themethod of claim 8, wherein the first direction is the mesial direction,and the second direction is the lingual direction.
 10. The method ofclaim 9, wherein the second direction is the labial direction.
 11. Themethod of claim 8, wherein the first direction is the distal direction,and the second direction is the lingual direction.
 12. The method ofclaim 11, wherein the second direction is the labial direction.